In 2006, astronomers working on the Supernova Cosmology Project, which scans the universe for supernovae, or exploding stars, noted something no one had ever seen before. An unidentified light appeared, grew increasingly brighter for about 100 days, and then over the next 100 days faded away completely.

Not a supernova

Sure, there are other things in the night sky that behave that way, including supernovae themselves, but this particular burst of brightness carried completely unrecognizable features. The light came to our attention thanks to the Hubble Space Telescope, which wanders the universe sending back pictures of very cool things for us to investigate. In this case, Hubble was trained on a cluster of galaxies in a constellation known as Bootes, which is home to Arcturus, said to be the fourth brightest star in the night sky. But this mystery light in 2006 briefly gave Arcturus more competition than a supernova.

A supernova gives off an intense amount of light that can actually outshine a galaxy during its relatively brief period of existence. But supernovae do not last longer than 70 days, and usually are radiant for only about three weeks. The mystery light, on the other hand, burned for 200 days. Another example of light changes in the night sky is microlensing, in which light from a distant object bends around something of enormous mass, such as a cluster of galaxies, thanks to gravitational pull. This light didn’t fit that description, either.

No signature matches

In fact, when astronomers compared its atomic spectra signature with all known signatures in their databases, they found nothing that matched. Spectral signature analysis is one way we determine the elemental makeup of something that is light years away. The signature results from a pattern of light wave frequencies, which begin thanks to the vibrations of electrons. Electrons within an atom can vibrate at different frequencies, so atoms emit light waves at different frequencies. We see those differences as bands of color when we look at them through a device that separates the wavelengths, and different elements exhibit different banding patterns: hydrogen, for example, has a different color banding pattern than helium. Thus, we can look at light traveling from billions of miles away and determine the elemental makeup of its source based on the banding patterns of these emissions.

But the banding signature of the mystery light didn’t match that of any known cosmic object. What also remained mysterious was exactly how far away the light was. Astronomers could roughly say that it was no closer than 130 light years because it lacked parallax motion, which diminishes with distance. For a good example of how parallax motion changes with distance, close one eye and hold your thumb up close to your face. Now, switch eyes, closing one and opening the other. Switch back. You’ll see that your thumb appears to shift position. That’s parallax motion. Now, hold your thumb at arm’s length and do the same thing. You’ll see that the shift is considerably less. At a predictable distance—130 light years—such a shift on a cosmic scale would be undetectable. But all astronomers can really say is that it’s no closer than that and no farther away than 11 billion light years, based on the lack of a hydrogen signature.

Destined to remain an engima

So right now, the Mystery Light (otherwise known as SCP 06F6) that’s not too close and, in cosmic terms, not that far away, remains an enigma. Some have suggested that it was the radiant remnant of an enormous interstellar collision, perhaps between a white dwarf, an incredibly dense star at the end of its life, and a black hole. But the signs—from the spectral signature to the lack of microlensing—don’t really fit that scenario. Whatever it was, this light that grew in brightness by 120 times before fading away completely will remain a mystery for the foreseeable future.